A fiber optic communication system in widespread use includes mainly a transmitter, an optical fiber corresponding to a transmission line, and a receiver.
At a point of connection between an optical waveguide type transmitter and an optic fiber, an optical signal transformed from an electrical signal is inputted from an optical waveguide within the transmitter to the optical fiber. The optical signal inputted to the optical fiber is then transmitted by the optical fiber. At a point of connection between an optical waveguide type receiver and the optic fiber, the transmitted optical signal is inputted to an optical waveguide of the receiver. The optical signal inputted to the optical waveguide is then inputted to a light receiving element such as a photo diode (PD), where the optical signal is transformed to an electrical signal.
In an optical transmission/reception module into which the above-described receiver and transmitter are modularized together, a plurality of optical devices are arranged on an optical waveguide within the module. The plurality of optical devices include a laser diode (LD) corresponding to a light source, a PD corresponding to a light receiving element, and the like.
In an optical transmission/reception module provided with such a configuration, out of an optical signal emitted from the light source, a light component not having been coupled into the optical waveguide becomes “stray light”. The stray light propagates outside or inside the substrate of the optical waveguide. In particular, the stray light propagating inside the substrate becomes a cause of increasing noise in the signal, as a result of its reentering the optical waveguide or entering the light receiving element connected to the optical waveguide.
In relation to this problem, technologies described in the following Patent Literature 1 to Patent Literature 3 have been known as methods for eliminating stray light generated as described above.
FIG. 6 is a diagram showing a cross section of an optical modulator described in Patent Literature 1. In Patent Literature 1, the optical modulator is described as one in which an optical waveguide 101 is formed on the top or bottom surface of a thin plate 102 and a stray light elimination means (light absorption section 103 in FIG. 6) is arranged within or close to the thin plate 102.
FIG. 7 is a diagram showing an optical IC (integrated circuit) described in Patent Literature 2. In the optical IC of FIG. 7, an optical waveguide 101 is formed on the top surface of a substrate 104, and a groove (triangular groove 105) running approximately in parallel with the optical waveguide 101 is formed at the bottom surface of the substrate 104 in a manner to be approximately opposed to the optical waveguide 101. Surfaces constituting the groove (triangular groove 105) are not perpendicular or parallel to the bottom surface of the substrate 104. Patent Literature 3 describes an optical device provided with a stray light reflection means for guiding stray light outside a thin plate on which an optical waveguide is formed.